Set a option for Chinese developers

[yjlc-pc]

1.Set the mobile preview's URL "google.com" to "tuna.tstinghua.edu.cn".
2.Set the Gradle download URL to Huawei's Server.
3.Set the git's URL to gitee.

[mhsmith]

I don't think we're interested in playing cat and mouse with the Chinese government's censors, or verifying the integrity of mirrors that aren't under our control. My understanding is that most Chinese software developers are familar with these issues, so they can either use a VPN, or change the URLs themselves.

In particular:

• The briefcase new command can take an alternative template URL on the command line.
• The briefcase create command can take a template URL from the project's configuration file.

[ColdWindScholar]

Yes , Github almost cannot connect in China